The Atom

THE ATOMSmallest particle of an element

that retains the properties of the element

THE BEGINNING Democritus first

suggested the existence of atoms in the 4th century BC.

John Dalton was the first to perform scientific experiments to test and correct his idea of the atom. He created the first

“Atomic Theory”

DALTON’S ATOMIC THEORY1. All elements are composed of tiny

indivisible particles called atoms.2. Atoms of the same element are

identical.3. Atoms of different elements can

combine physically or chemically (in whole number ratios to form compounds).

4. Chemical reactions occur when atoms are separated, joined, or rearranged.

However, not all holds true today…

THE ATOM DIVERSIFIES… Electrons – negatively charged

subatomic particlesDiscovered by J.J. Thomson in 1897

Conducted Cathode Ray Experiment

ELECTRON continuedRobert Millikan measured the quantity of

charge and the ratio of charge to mass of the electron Calculated the electron’s mass Oil Drop Experiment

THINGS WE KNOW ABOUT CHARGES Atoms have no net electric charge Electric charges are carried by particles

of matter Electric charges always exist in whole

number ratios When a given number of negatives

combines with the same number of positives, a neutral atom is formed.

So…we know there must be something POSITIVE inside the atom.

PROTONS Goldstein (1886) used cathode ray

tubeobserved a ray in the opposite direction

(canal rays)named particle the proton

Its mass was ~ 1840 times the mass of the electron

NEUTRONS Chadwick (1932) confirmed the

existence of the neutron Approx. same mass as the proton

STRUCTURE OF THE ATOM Thomson’s

“Plum Pudding” modelElectrons are

interspersed throughout positive material

PLUM PUDDING UNDER ATTACK Rutherford

tested Thomson’s theory

Gold Foil Experiment

GOLD FOIL EXPERIMENT

GOLD FOIL EXPERIMENT EXPLAINED Beam of Alpha particles directed at

gold foil. Particles were scattered suggesting a

dense mass of positive charge Rutherford proposed the idea of the

atomic nucleus.Most of the atom is empty spaceNucleus contains protons and neutronsSpace contains electrons

WHERE DOES THAT LEAVE US? In the atom, we have:

Protons (p+)Neutrons ( n0)Electrons (e-)

Protons and Neutrons are in the nucleus. Electrons are outside the nucleus.

Identities of elements are determined by the number of protons in the nucleus.

THE ATOM BY THE NUMBERS Atomic Number – number of protons

(whole number) Mass Number – Protons plus Neutrons

(number of particles in the nucleus) Atomic Mass – weighted average of

the masses of all isotopes of the element (decimal number) 6

C12.011

SAME ELEMENT…DIFFERENT NUMBERS Isotope – atoms of the same element

with different numbers of neutronsAtomic Number will be the SAMEMass Number will be DIFFERENTAtomic Mass will be the SAME

In an Isotope,Protons – SAMENeutrons - DIFFERENTElectrons – SAME

Ex. Carbon – 12 and Carbon - 14

PROTONS, NEUTRONS, ELECTRONS AND THE ATOM

Element Atomic

Number

Mass Numb

er

Protons

Neutrons

Electrons

9 10

14 7

21 20

13 27

56 26

Carbon-13

Beryllium-10

Neon-20

Boron-11

Sulfur-33

PROTONS, NEUTRONS, ELECTRONS AND THE ATOM

Element Atomic

Number

Mass Numb

er

Protons

Neutrons

Electrons

Fluorine-19 9 19 9 10 9

Nitrogen-14

7 14 7 7 7

Calcium-41 20 41 20 21 20

Aluminum-27

13 27 13 14 13

Iron-56 26 56 26 30 26

Carbon-13 6 13 6 7 6

Beryllium-10

4 10 4 6 4

Neon-20 10 20 10 10 10

Boron-11 5 11 5 6 5

Sulfur-33 16 33 16 17 16

ATOMS VS. IONS Atoms can also gain and lose electrons. An atom that has lost or gained an

electron is an ION. Positive Ions (CATIONS) form when

atoms lose electronsEx. Calcium-41 atom 20 p+ 20 e- 21 n0

Calcium-41 +2 ION 20 p+ 18 e- 21 n0

Negative Ions (ANIONS) form when atoms gain electrons. Ex. Oxygen-16 atom 8 p+ 8 e- 8 n0

Oxygen-16 -2 ION 8 p+ 10 e- 8 n0

IONS In an ION,

Protons – SameNeutrons – SameElectrons – Different

More electrons means a NEGATIVE ion.Anion (O2-)

Less electrons means a POSITIVE ion.Cation (Ca2+)